The present invention relates to an image forming apparatus such as a copying machines, a laser printer and a facsimile, and relates to a image forming method.
Generally, image density of toner images in an image forming apparatus may be changed due to disturbances such as an environmental change in temperature or humidity around the apparatus or a change in durability of the apparatus.
In order to stabilize the image density, one of conventional image forming apparatuses forms test toner images (hereinafter referred to as “detection patterns”) on a photoconductor, so that a developing bias, which affects the image density, is optimized on the basis of densities of the detection patterns (see JP 2004-61809 A).
Specifically, the developing bias is variously changed and set while the detection patterns are formed on the photoconductor and transferred onto an intermediate transfer body. Then, a density sensor detects the densities of the detection patterns on the intermediate transfer body. A desired image density is obtained by adjusting the developing biases so that one of the detected densities reaches a preset target density.
When densities of detection patterns are low, irregularity in densities becomes pronounced. Under harmful influence of the density irregularity, outputs from the density sensor may be greatly varied, so that it becomes impossible to accurately detect the densities of the detection patterns. To enhance the detection accuracy, densities of plural detection patterns are detected to obtain an average of the densities, while lengths of the detection patterns are set to be longer than a peripheral length of the photoconductor.
In the conventional image forming apparatus, however, there have been a problem that the amounts of waste toner and toner consumption are increased, and a problem that time taken to detect the densities of detection patterns becomes long because each length of the detection patterns is longer than the peripheral length of the photoconductor.